The present application claims priority to Japanese Patent Application No. 2001-248436 filed Aug. 20, 2001, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a reflective reading optical system, and more particularly, to a reflective reading optical system suitable for an image reading apparatus (particularly a digital copying machine, a facsimile or a scanner) equipped with a one-dimensional imaging element such as a line CCD (charge coupled device), for example.
2. Description of the Related Art
The optical surfaces having a focal power in the multi-beam type scanning optical system proposed by Japanese Laid-Open Patent Application 2000-275557 all consist of reflective surfaces. In this construction, the increase in the width of the beam along the image height due to positional offsetting for each beam is eliminated via the reflective surfaces. An imaging optical system using reflective surfaces is also known. For example, Japanese Laid-Open Patent Application H11-23971 proposes a device that includes both reflective and transmitting surfaces. However, because the device disclosed in this laid-open patent application is used in order to obtain two-dimensional images, correction of distortion is not adequate. By contrast, adequate distortion correction is possible in a reading optical system used to obtain one-dimensional images, and a device using reflective surfaces is also known.
However, because a light source having a wide wavelength range is used in an image reading apparatus, if a transmitting surface having a focal power is included in the reading optical system, a reduction in contrast caused by color aberration is unavoidable. In addition, if multiple reflective surfaces are to be used in a reading optical system, the reflective surfaces must be decentered, and as a result of non-symmetricity of light beam positions in the decenter cross-section, the problem arises that aberration correction becomes difficult.
A main object of the present invention is to provide a reflective reading optical system by which high contrast is obtained even if the light source has a wide wavelength range, and by which aberrations are well-corrected even where the reflective surfaces are decentered.
In order to attain the above object, the reflective reading optical system of the present invention is a reflective reading optical system that forms the image of the original document on an one-dimensional imaging element, such optical system having as optical surfaces with a focal power multiple reflective surfaces only, wherein at least one of such surfaces is a free-form surface that has a symmetrical surface relative to the main scanning direction and has no symmetrical surface relative to the sub scanning direction, and wherein each reflective surface is arranged such that they are tilted relative to the Y-axis.
Here, the Z-axis extends along the incident light beam, which is the axial main light beam that passes through the center of the original document and the center of the aperture and strikes the first reflective surface. The Y-axis extends along the length of the one-dimensional imaging element. The X-axis extends perpendicular to the Y-axis and the Z-axis.
It is also acceptable if an aperture exists between the first reflective surface and the last reflective surface struck by the axial main light beam.
It is also acceptable if the aperture is constructed integrally with a reflective surface.
It is also acceptable if, when the length of the aperture extending in the main scanning direction is SY and the length thereof extending in the sub scanning direction is SX, the relationship expressed by the condition SY greater than SX is satisfied.
It is also acceptable if the path of the axial main light beam that first strikes the reflective surface and the path of the axial main light beam that strikes the imaging element are essentially perpendicular to each other on the X-Z cross-section.
The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description taken in conjunction with the accompanying drawings.